1. Field of the Invention
The present invention relates generally to a mobile communication system using an orthogonal frequency division multiple access (OFDMA) scheme (hereinafter, referred to as an OFDMA mobile communication system), and in particular, to an apparatus and a method for adaptively assigning subchannels based on feedback channel quality information (CQI) of each subchannel.
2. Description of the Related Art
With the introduction of the cellular mobile communication system in the U.S. in the late 1970's, Korea began to provide a voice communication service in a first generation (1G) analog mobile communication system, AMPS (Advanced Mobile Phone Service). In the mid 1990's, Korea deployed a second generation (2G) mobile communication system, a CDMA (Code Division Multiple Access) system, to provide voice and low-speed data services.
In the late 1990's, Korea partially deployed a third generation (3G) mobile communication system, the IMT-2000 (International Mobile Telecommunication-2000 system), aiming at an advanced wireless multimedia service, worldwide roaming, and a high-speed data service. The 3G mobile communication system was especially developed to transmit data at a high data ate and in compliance with the rapid increase in volume of the serviced data.
The 3G mobile communication system is evolving to a fourth generation (4G) mobile communication system. The 4G mobile communication system is currently under the standardization process for the purpose of providing an efficient interworking and an integrated service between a wired communication network and a wireless communication network beyond the simple wireless communication service which the previous-generation mobile communication systems provide. It follows that a technology for transmitting a large volume of data at up to a capacity level available in the wired communication network must be developed for the wireless communication network.
In this context, studies are being actively conducted to utilize the orthogonal frequency division multiplexing (OFDM) scheme as a useful scheme for high-speed data transmission on wired/wireless channels in the 4G mobile communication system. OFDM is a special case of multi-carrier modulation (MCM) in which a serial symbol sequence is converted to parallel symbol sequences and modulated to a plurality of mutually orthogonal sub-carriers (or sub-carrier channels).
The first MCM systems appeared in the late 1950's for military high frequency (HF) radio communication, and OFDM with overlapping orthogonal sub-carriers was initially developed in the 1970's. In view of the orthogonal modulation between the multiple carriers, the OFDM has limitations in actual implementation for systems. In 1971, Weinstein, et. al. proposed an OFDM scheme that applies a DFT (Discrete Fourier Transform) to the parallel data transmission as an efficient modulation/demodulation process, which was a driving force behind the development of the OFDM. Also, the introduction of a guard interval and a cyclic prefix as the guard interval further mitigates adverse effects of the multi-path propagation and the delay spread on the systems. That is why OFDM has been widely exploited for digital data communications such as digital audio broadcasting (DAB), digital TV broadcasting, wireless local area network (WLAN), and wireless asynchronous transfer mode (WATM).
Although the complexity of hardware was an obstacle to the wide use of the OFDM, recent advances in digital signal processing technology including FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform) enable the OFDM to be implemented. OFDM, similar to FDM (Frequency Division Multiplexing), boasts of an optimum transmission efficiency in a high-speed data transmission because it transmits the data on sub-carriers, maintaining an orthogonality among them. The optimum transmission efficiency is further attributed to a good frequency use efficiency and a robustness against the multi-path fading in the OFDM.
Overlapping frequency spectrums lead to an efficient frequency use and a robustness against frequency selective fading and multi-path fading. The OFDM reduces the effects of the ISI (Inter Symbol Interference) by use of guard intervals and enables the design of a simple equalizer hardware structure. Furthermore, since the OFDM is robust against impulse noise, it is increasingly popular in communication systems.
In an OFDM-based multiple access scheme (OFDMA), a plurality of users share the subcarriers of one OFDM symbol. OFDMA communication systems include these defined in IEEE (Institute of Electrical and Electronics Engineers) 802.16a and IEEE 802.16e. The IEEE 802.16a communication system is a broadband wireless access (BWA) communication system based on OFDMA.
The IEEE 802.16e communication system is an expansion of the IEEE 802.16a communication system, which supports the mobility of the mobile station or user equipment. Both the IEEE 802.16a and IEEE 802.16e communication systems use a 2048-point IFFT and 1702 subcarriers. From among the 1702 subcarriers, 166 of the subcarriers are pilot subcarriers and the remaining 1536 subcarriers are data subcarriers. The 1536 data subcarriers are divided into 32 subchannels, each subchannel having 48 subcarriers, and the subchannels are assigned to a plurality of users according to system situations.
“Subchannel” refers to a channel comprising a plurality of subcarriers. 48 subcarriers form one subchannel in the IEEE 802.16a and IEEE 802.16e communication systems. The OFDMA mobile communication systems aim to achieve a frequency diversity gain by distributing all of the available subcarriers, particularly the data subcarriers, over the whole frequency band.
A scheme for dynamically changing subcarriers assigned to a particular user is referred to as frequency hopping (FH). A combination of the FH and the OFDMA is referred to herein as FH-OFDM. A communication system using the FH-OFDM scheme (hereinafter, referred to as an FH-OFDM communication system) hops the frequency band of the subcarriers assigned to the users by the FH. The FH-OFDM communication system also seeks to achieve a frequency diversity gain by distributing all of the available subcarriers, particularly the data subcarriers, over the whole frequency band.
As described above, the IEEE 802.16a and IEEE 802.16e communication systems divide a wide band of, for example 10 MHz, into subchannels in the frequency domain only. They use the 1702 subcarriers per OFDM symbol and the 2048-point IFFT. Therefore, if the subchannels are assigned using a Reed-Solomon (RS) sequence having a relatively good collision characteristic between the subchannels under a multicell environment, about 40 cells can be identified since 41×40=1640.
However, to facilitate a network designing along with the development of the mobile communication systems, up to at least 100 cells need to be identified. In terms of the number of identifiable cells, the OFDMA has limitations because it forms the subchannels only in the frequency domain.
A Flash-OFDM scheme using a narrow band of 1.25 MHz defines 113 hopping sequences as a basic resource assignment unit. The 113 hopping sequences hop different subcarriers for one period of 113 OFDM symbols using a 128-point IFFT. A communication system using the Flash-OFDM scheme (hereinafter, a Flash-OFDM communication system) can identify 113 different cells by defining the different hopping sequences for the respective 113 cells in network designing. The Flash-OFDM scheme is, however, viable for only a narrow band, which implies that it cannot contribute to a capacity increase needed for the current 4G communication system.
In a typical cellular communication system, a signal transmitted from a transmitter travels to a receiver from multiple paths. Thus, the received signal experiences frequency-selective fading. With reference to FIG. 1, the frequency response characteristics in relation to frequency-selective fading in an OFDMA mobile communication system will be described below.
FIG. 1 illustrates transmit frequency response characteristics and receive frequency response characteristics in an OFDMA mobile communication system.
Referring to FIG. 1, the frequency spectrum 111 of an OFDMA signal transmitted from a transmitter, for example in a base station (BS), has the same frequency responses for the respective subcarrier signals in the entire frequency band. Since each subcarrier signal in the frequency band has the same frequency response in the OFDMA mobile communication system, the global frequency band exhibits the same frequency response characteristic. It is assumed that the subcarrier signals having the same frequency response are transmitted to different users (i.e. mobile subscriber stations: MSSs), for example a first MSS and a second MSS. While obviously the subcarriers are transmitted to more MSSs in the OFDMA mobile communication system, it is assumed that the entire subcarrier signals are equally transmitted to the two MSSs, in order to compare their frequency response characteristics.
The first MSS receives the entire subcarrier signals from the BS, with a different frequency spectrum 121 from the frequency responses of the transmitted subcarrier signals. Some subcarriers have frequency responses at or above a demodulation threshold and others have frequency responses below the demodulation threshold.
As illustrated in FIG. 1, four subcarrier signals have frequency responses below the threshold from among the total received signals at the first MSS. When the BS transmits data to the first MSS on the four subcarriers, the first MSS cannot normally receive the data because of the frequency-selective fading.
The second MSS also receives the entire subcarrier signals from the BS, with a different frequency spectrum 131 from the frequency responses of the transmitted subcarrier signals. Some subcarriers have frequency responses at or above the threshold and others have frequency responses below the threshold. As illustrated in FIG. 1, five subcarrier signals have frequency responses below the threshold from among the total received signals at the second MSS. When the BS transmits data to the first MSS on the five subcarriers, the second MSS cannot normally receive the data because of the frequency-selective fading.
Consequently, some of the total subcarriers are feasible for a particular MSS, and others are not in the OFDMA mobile communication system.
In this context, a frequency-selective adaptive modulation and coding (AMC) scheme was proposed to compensate for the performance degradation caused by the frequency-selective fading. The frequency-selective AMC scheme adaptively determines a modulation and a coding method for each subcarrier according to the frequency response of the subcarrier. The modulation scheme adjusts the transmit power and the coding scheme adjusts a coding rate. The frequency response can be defined as a carrier-to-interference and noise ratio (CINR). Then, the modulation and coding method is adaptively determined for each subcarrier according to the CINR of the subcarrier.
The frequency-selective AMC scheme involves a plurality of modulation schemes and a plurality of coding schemes. It selects a combination of a modulation scheme and a coding scheme, for modulation and coding. Modulation and coding combinations are called modulation and coding schemes (MCSs). Level 1 to level n MCSs can be defined depending on the number of available MCSs. Thus, the frequency-selective AMC scheme adaptively selects an MCS level according to the frequency response characteristics of a BS and a MSSs, to thereby increase the transmission capacity of the BS and improve the total system efficiency of the OFDMA mobile communication system.
The use of the AMC scheme in the OFDMA mobile communication system requires a knowledge of the frequency response of each subcarrier. That is, the BS can apply the AMC scheme only when the MSSs feed back the CINRs of all of the subcarriers to the BS. With reference to FIGS. 2A and 2B, the feedback of the frequency response (i.e. CQI (Channel Quality Indicator)) of each subcarrier will be described below.
FIG. 2A illustrates the transmission positions of pilot signals in the frequency domain in the typical OFDMA mobile communication system.
Referring to FIG. 2A, the transmitter (i.e. the BS) transmits a pilot signal only on pilot subcarriers whose positions are preset in the OFDMA mobile communication system. The receiver (i.e. the MSS) already has knowledge of the positions of the pilot subcarriers and knowledge of the pilot signal on the pilot subcarriers. The pilot signal is a preset sequence between the BS and the MSSs. The MSS detects the CINR of each pilot subcarrier by dividing a signal received on the pilot subcarrier by the pilot signal of the pilot subcarrier. Then it interpolates the CINRs of the pilot subcarriers, thereby estimating the CINRs of the data subcarriers. The MSS feeds back the CINRs (i.e. CQIs) of the subcarriers to the BS and the BS selects an MCS for the corresponding subcarriers based on the CQIs.
FIG. 2B illustrates the transmission positions of the pilot symbols in the time domain in the typical OFDMA mobile communication system.
Referring to FIG. 2B, the BS transmits a pilot symbol during a predetermined symbol period. Although each BS uses the same subcarriers, the BS generates a pilot symbol by multiplying a block, having as many subcarriers as a predetermined spreading factor, by an orthogonal code of the same length, and multiplying the resulting product by a different PN (Pseudo Noise) code from those of other BSs. The MSS identifies the signal from the BS by the PN code, calculates the CINR of each subcarrier using the orthogonal code, and feeds back the CQI of the subcarrier to the BS.
The above-described CQI feedback method is suitable only under the assumption that the channel states of the subcarriers assigned to the MSS is unaltered in the OFDMA mobile communication system. However, the current 4G mobile communication system considers the adoption of the OFDMA for high-speed mobile communications. Therefore, the above assumption is not valid. That is, once the subcarriers are assigned to an MSS, their channel states vary. Thus, the MSS must feed back the changing CQI of each of the subcarrier to the BS so that the BS can accurately use the AMC scheme.
Yet, the frequent CQI feedback for each subcarrier brings about a signaling overhead and the CQI feedback signaling acts as an uplink interference.
Therefore, there is a need for a method for efficiently using the AMC scheme, supporting the high-speed mobile communication service in the OFDMA mobile communication system.